The barium titanate powder has been widely utilized to produce multi-layer ceramic chip capacitors (MLCC), positive temperature coefficient thermistors, resistors, and the like. It is well-known that barium titanate powder can be manufactured via solid state reaction of barium carbonate (BaCO3) and titanium dioxide (TiO2) at high temperature. As the trend in MLCC (multi-layer ceramic chip capacitor) continues towards further and further miniaturization with large capacity, calcination at a low temperature, high frequency, and volumetric efficiency, the demand for not only finer and more uniform barium titanate powders has increased tremendously, but the need for purity and distribution has also escalated as well. Thus, various liquid state reaction methods such as hydrothermal method, co-precipitation (oxalate) method, and alkoxide method have been developed to produce barium titanate powders satisfying these characteristics.
The oxalate method is well discussed by W. S. Clabaugh et al. in Journal of Research of the National Bureau of Standards, Vol. 56(5), 289–291(1956) to produce barium titanate by precipitating barium titanyl oxalate with addition of a mixture solution containing Ba and Ti ions to an oxalic acid. However, this method has several drawbacks: (i) it is difficult to control particle size and stoichiometric mole ratio of Ba to Ti; (ii) hard aggregates between particles are formed during the pyrolysis, thus requiring strong pulverization to remove these hard aggregates; (iii) because extremely fine particles are enormously produced during the strong pulverizing, it is hard to disperse the powder for forming and abnormal grain growth occur during sintering process. Thus, the barium titanate powders produced in this manner are not adequate for the applications to multilayer ceramic capacitors.
Therefore, a hydrothermal method has been recently given attention to because of the trend of thinner and higher layered dielectric layer in MLCC. However, This method has disadvantages, like high manufacturing cost and complex process, due to use of autoclave, in spite of its high product quality. Therefore, there are increasing demands for developing simpler methods for preparing barium titanate powders in low price to be competitive in the market.
Inventors of the present invention have filed patent applications in South Korea (Korea Patent Application Nos. 2000-46125 and 2001-9066) for the preparation of barium titanate powder which exhibit improved yield with shortened reaction time and optimized stoichiometry of barium to titanium. The method for preparing barium titanate powder disclosed in the above Korea Patent applications comprises the steps of: precipitating barium titanyl oxalate (BaTiO(C2O4)24H2O) by spraying an aqueous mixture of barium chloride (BaCl22H2O) and titanium tetrachloride (TiCl4) to an aqueous solution of oxalic acid, via a nozzle in high speed and aging, filtering and washing the same; pulverizing the obtained barium titanyl oxalate, drying, and pyrolizing to produce barium titanate (BaTiO3) powder; and re-pulverizing the pre-pulverized barium titanate powder. In the pulverizing process, it can be performed by methods such as dry pulverization using an atomizer and jet mill or wet pulverization using a ball mill, planetary mill, and beads mill. In case it is needed to blend barium titanate (BaTiO3) with other metal additives in the pulverizing step, wet pulverization is more preferable. Among wet pulverizing instruments, planetary mill or ball mill is suitable for laboratory scale, while a beads mill is for industrial scale.
However, when the beads mill is employed for the wet pulverization of barium titanyl oxalate it causes the following problems:
(1) Impurities present in the solution occlude inside barium titanyl oxalate particles during the precipitating step. Thus, impurities, such as oxalic acid or chloride ion remain in barium titanyl oxalate prepared by co-precipitation (oxalate) method, no matter how many it is washed. Since this barium titanyl oxalate mixed with water has pH around 3, it may cause corrosion of pre-mixer whose material is stainless steel. Further, it is difficult to control exothermic heat in case of using acid-resistant material such as polyurethane. Therefore, it requires a use of expensive material like titanium as a pre-mixer. After the wet pulverization, the pH of the barium titanyl oxalate slurry becomes 2 which is lower than that before the wet pulverization. Thus, it may decrease material's durability of beads mill (inside part of mill chamber) like zirconia and polyurethane. Furthermore it causes corrosion of a dryer.
(2) The presence of chloride ions in the barium titanyl oxalate will result in the formation of a BaCl2 liquid phase during sintering, which is disclosed in Journal of Inorganic chemistry, vol. 9(11) 2381˜89(1970). Thus, the calcined barium titanate aggregates more and exhibits poor dielectric characteristics.